When you click Play on Phone, Voice Reply, Voice Forward, or Call Sender on the Messaging toolbar, the system displays a pop-up window where you need to specify which phone is to be used for the requested action. The Play on Phone settings on the User Preferences General page provide options to manage this pop-up as follows:
Always use this phone: Use this option to specify a phone number that you want to use for Play on Phone, Voice Reply, Voice Forward, or Call Sender. This phone number will automatically be used every time it is needed, and you will not be prompted with a pop-up for this purpose.
Ask me every time which phone to use: Use this option to restore the default setting where the system displays a pop-up that prompts you to select a phone each time you use Play on Phone, Voice Reply, Voice Forward, or Call Sender.